Why Does an MRI Make So Much Noise?

Why Does an MRI Make So Much Noise? Understanding the Sounds

MRI stands for magnetic resonance imaging and is one of the most powerful medical imaging tools available today. MRI scans produce detailed images of the inside of your body. But why does an MRI make so much noise?

The reason is to do with how MRI technology works. Here, we will explore how MRI scanners operate to answer the question, “why are MRI scans so loud?”.

How MRI machines operate

MRI works by creating a strong magnetic field around an individual and passing radio waves into their body.

The strong magnetic field is created by powerful magnets found in the tube-shaped structure of the MRI machine called superconducting magnets. These magnets are made of coils of superconducting metal wires. ‘Superconducting’ means that when the metal wires are cooled to a low enough temperature, they put up no resistance to the movement of electrical current through them.

During an MRI scan, the individual to be scanned lies on an examination table that is slid into the centre of the MRI machine. The individual is surrounded by the magnetic field generated by the powerful MRI magnets.

The magnetic field works with the water molecules present in all tissues. Specifically, it affects the protons in the hydrogen atoms of water molecules. It causes these protons to become aligned to the magnetic field, that is, to all point in the same direction.

When bursts of radio waves are passed through the human body, the protons are knocked out of alignment. When the radio waves stop, the protons realign and in doing so, release energy that is detected by the MRI machine.

These energy signals are then converted into electrical signals, which are processed by a computer to create a 3D MRI scan. Areas of the body with high amounts of water (eg blood, cerebrospinal fluid, etc) appear brighter and whiter on an MRI scan. Meanwhile, areas that are more dense, that is, have less water, appear darker.

The reason behind MRI noises

An MRI machine makes several different noises. These MRI noises are caused by different parts of the machine, namely the coldhead and the gradient coil. To understand how these parts make different sounds it helps to understand some of the physics of MRI sound production.

Sound of the coldhead

As mentioned earlier, MRI uses superconducting magnets. In order for these magnets to work properly, they must be kept at a very low temperature (around -270℃). This is made possible by immersing the magnets in liquid helium.

As the scanner generates a lot of heat, some of the liquid helium boils and turns into helium gas. As an MRI machine is always switched on for the duration of its lifetime – whether or not it is performing a scan – liquid helium is constantly being lost as helium gas.

To maintain the levels of liquid helium needed to run an MRI machine, the coldhead turns this helium gas back into liquid helium. As it does so, the coldhead expands and contracts. This is what causes the MRI machine to make a constant whirring or chirping sound. This sound is always present, even when a scan is not being performed.

Sound of the gradient coil

The sound produced by the gradient coil is the main sound you will hear during your MRI. This is a loud banging or knocking sound.

The gradient coil comprises several metal coils that work alongside the superconducting magnets.

The superconducting magnets create a constant, strong magnetic field while the gradient coil creates temporary magnetic fields that affect the main magnetic field. In doing so, the gradient coil provides spatial information for the MRI signals produced during your scan. This means we can calculate where in your body the signals are coming from to create accurate images.

The temporary magnetic fields are produced by switching the electrical current through the metal coils on and off. This makes the coils vibrate. As it expands and contracts quickly, the fixtures within the MRI machine are placed under strain. This mechanical stress is what causes the loud banging noises during a scan.

Loudest MRI sequences explained

Some types of MRI scans produce louder noises than others. This is due to the pattern of switching the electrical current on and off within the gradient coils. The specific gradient coil pattern (sequence) affects how loud the MRI scan will be.

The loudest MRI scans use a sequence called echo-planar. This is used in several different kinds of MRI applications, including cardiac (heart) MRI scans and magnetic resonance angiography (looking at blood vessels).

MRI noise vs. other medical scans

When compared to other medical imaging tests, such as CT scans, X-rays and ultrasound scans, MRI produces the loudest noises.

A CT scan or X-ray, which use ionising radiation, both produce quiet whirring noises as the machines work. An ultrasound scan produces hardly any sound at all.

However, despite MRI being the loudest medical imaging tool, it produces the highest resolution images.

Tips for patients to manage MRI noise

If you are concerned about loud MRI noises, there are ways you can help ease your anxiety:

Listen to music

You can ask for special headphones that can be worn in an MRI machine so you can listen to music. Although music will not be able to completely mask the loud sounds of the MRI machine, it can help focus your mind away from these noises.

Wear ear plugs

You will be given ear protection in the form of ear plugs and/or headphones, which you will wear during your MRI scan. This will help reduce the volume of the noises you will hear.

Practice mindfulness

Mediation, deep breathing, visualisations and other mindfulness techniques can help you tune out the noise of the MRI machine and remain calm.

Speak to your care team

Let your care team know prior to your scan that you are feeling anxious about the MRI noises so they can better support you. During your scan, you can squeeze a call buzzer and talk to your radiographer at any time through the two-way intercom built into the MRI machine. If you find that you can no longer tolerate the noise, you can squeeze the call buzzer and use the intercom to ask your radiographer to stop the scan.